Reactivity of dissolved- vs. supercritical-CO2 phase toward muscovite basal surfaces

Abstract:

The current understanding of geochemical reactions in reservoirs for geological carbon sequestration (GCS) is largely based on aqueous chemistry (CO₂ dissolves in reservoir brine and brine reacts with rocks). However, only a portion of the injected supercritical (sc) CO₂ dissolves before the buoyant plume contacts caprock, where it is expected to reside for a long time. Although numerous studies have addressed scCO₂-mineral reactions occurring within adsorbed aqueous films, possible reactions resulting from direct CO₂-rock contact remain less understood. Does CO₂ as a supercritical phase react with reservoir rocks? Do mineral react differently with scCO₂ than with dissolved CO₂? We selected muscovite, one of the more stable and common rock-forming silicate minerals, to react with scCO₂ phase (both water-saturated and water-free) and compared with CO₂-saturated-brine. The reacted basal surfaces were analyzed using atomic force microscopy and X-ray photoelectron spectroscopy for examining the changes in surface morphology and chemistry. The results show that scCO₂ (regardless of its water content) altered muscovite considerably more than CO₂-saturated brine; suggest CO₂ diffusion into mica interlayers and localized mica dissolution into scCO₂ phase. The mechanisms underlying these observations and their implications for GCS need further exploration.